Crypt7 and crypt8 decryption
Shortly, you have different ways to get down to manage the new Android archive msgstore.db.crypt12.
You just bought a brand-new Android device, so no WhatsApp message message has been received yet on Android
In this case you just import your iPhone's messages only, by NOT ticking "Merge Android archive" in WazzapMigrator (default behaviour)
You DON'T HAVE root but you do want to merge iPhone message with pre-existing Android messages
[update April 2018] currently not possible on Android >=7 without root, see issue #10 on Github project
extract the decrypted msgstore.db using WhatsApp-Key-DB-Extractor on your PC (it extracts the undecrypted msgstore.db from your msgstore.db.crypt12)
after that you can use WazzapMigrator as usual (just make sure to select msgstore.db as source Android archive)
You HAVE root and want to merge iPhone message with pre-existing Android messages
Make sure to have rooted your Android device then copy /data/data/com.whatsapp/databases/msgstore.db to WhatsApp/Databases folder. You can rename it msgstore-unencrypted.db for easier recognition. After that you can use WazzapMigrator as usual, just make sure to select "msgstore-unencrypted.db" as source Android archive.
What is root: Rooting is a very technical procedure whose aim is to gain full administrative access to your device. If you want more info you can take a look here or here (please note: those links are not supported by WazzapMigrator in any way, they're provided for your reference only)
TECHNICAL DETAILS: This is due to Whatsapp recently changing its encryption from .crypt to .crypt6-7-8-9-10 (in just a few months). With .crypt6-7-8-9-10-11-12 they decided to periodically change the encryption key plus storing it in a private area of your phone, therefore not accessible without root access. Please note that NO app can decrypt the msgstore.db.crypt6-12 without prior key extraction from PC: this is due to Android security itself. I also don't see it possible for the foreseeable future since breaking such encryption would mean breaking the Android security (which would be a pretty big deal itself :D).